The 500-year-old mines of Bolivia's Cerro Rico mountain produced the silver that once made the Spanish empire rich. Now riddled with tunnels, the mountain is a death trap for the men and boys who work there - and who pray to the devil to keep them safe.

In a dingy tunnel, 15-year-old Marco shovels rocks into a wheelbarrow. Covered in dust and sweat, he's expected to carry 35 to 40 loads to the surface during a five-hour shift, often working at night so he can go to school during the day.

Marco's mother and her four children moved to Cerro Rico, the Rich Hill, after their father abandoned them. They live at the entrance of a tunnel, without any running water, using an abandoned mine as a bathroom.

"I want to be something better, not work in the mine… I'd like to get a degree, to be a lawyer," he says. But for now the family would not survive without his earnings.

During the Spanish Colonial era, two billion ounces of silver was extracted from the mountain. Over the same period about eight million people are estimated to have died, earning Cerro Rico the nickname, the Mountain that Eats Men.

Today about 15,000 miners work on the mountain, and the local widows' association says 14 women are widowed each month. Average life expectancy is 40.

Like everyone who works there, Marco worries about accidents, and about silicosis, a disease caused by breathing dust. His brother-in-law was only in his mid-20s when he died of it, Marco says.

"You eat the dust, it goes into your lungs and attacks you," says Olga, a single mother whose job is to guard equipment for the men running the mine by her house.

Her sons, Luis, aged 14, and Carlos, 15, work underground, pushing wheelbarrows like Marco, sometimes starting at 02:00 in order to fit in an eight-hour shift before school.

They have experienced another of the mountain's hazards - toxic gas released by the rocks.

"Your feet get weaker and you get a headache," explains Carlos. "The gas is what is left behind after the dynamite explodes."

Image caption
Luis chews coca before work

One woman told me her husband had died after breathing in the gas - he began feeling dizzy and fell down a mine shaft.

The high death toll on the mountain fuels superstition.

The men and boys all chew coca leaves, saying it helps filter the dust. They also make offerings of these coca leaves along with alcohol and cigarettes to El Tio - the devil god of the mines.

Each of the 38 businesses running mines on the mountain has a statue of El Tio in their tunnels.

"He has horns because he is the god of the depths," says Grover, Marco's boss.

"Usually we gather here on Fridays to make offerings, in gratitude because he gave us lots of minerals, and so that he will protect us from accidents.

"Outside the mine we are Catholics, and when we enter the mine, we worship the devil."

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Shockingly, Marco and Luis are far from the youngest children working in the mines.

"There are 10 children who I see when they come here, they have blisters on their hands, so I think they've been inside the mines. Children from eight, nine, 10... " says Nicolas Marin Martinez, headmaster of the only school on the mountain, run by the Swiss charity, Voix Libres.

A recent change in the law means children as young as 10 can now legally work in Bolivia, but not in the mines, which are considered too dangerous. So it was and remains illegal, it's just that the law is rarely enforced.

A report by Bolivia's ombudsman estimates that 145 children are working as miners, of whom 13 are 14 or younger. Another estimate puts the number of children employed on the mountain - including those sorting mineral ores outside the mines and helping guard machinery - at 400.

Image caption
Marco has been working in the mine for a year

Bolivian President Evo Morales is seeking a historic third term in elections on 12 October.

He's promised to give the wealth of the land back to the people - and the IMF says Bolivia has reduced poverty and nearly tripled income per capita since Morales has been in power.

But for the poorest of the poor living on Bolivia's richest mountain, it seems they have yet to benefit.

Image caption
View of the city of Potosi from the Cerro Rico

Catharina Moh's documentary for Our World will be broadcast on BBC World News on Friday and Saturday. Catch up later on the BBC iPlayer.